diff --git a/hermes_cli/plugin_validate.py b/hermes_cli/plugin_validate.py
new file mode 100644
index 00000000000..74d0849a051
--- /dev/null
+++ b/hermes_cli/plugin_validate.py
@@ -0,0 +1,434 @@
+"""``hermes plugins validate`` — admission checks for a plugin directory.
+
+This is the command the plugin-catalog admission CI (and the
+``.github/actions/plugin-validate`` composite action) runs against a
+candidate plugin. It performs static manifest checks plus a
+subprocess-isolated capability probe: the plugin is imported and its
+``register(ctx)`` called against a minimal recording stub context in a
+scratch child process (with a throwaway ``HERMES_HOME``), so a crashing or
+malicious plugin cannot take down the CLI, and the *actually registered*
+tools/hooks/middleware are compared against the manifest's declared
+``provides_*`` lists.
+"""
+
+from __future__ import annotations
+
+import json
+import os
+import re
+import subprocess
+import sys
+import tempfile
+from dataclasses import dataclass, field
+from pathlib import Path
+from typing import Any, Dict, List, Optional, Tuple
+
+_UPPER_SNAKE_RE = re.compile(r"^[A-Z][A-Z0-9_]*$")
+_CONFIG_TYPES = {"str", "bool", "int"}
+_PROBE_TIMEOUT = 30
+_PROBE_SENTINEL = "HERMES_VALIDATE_JSON:"
+
+
+@dataclass
+class ValidationReport:
+    """Result of validating one plugin directory."""
+
+    checks: List[Tuple[str, bool, str]] = field(default_factory=list)
+    warnings: List[str] = field(default_factory=list)
+
+    @property
+    def failures(self) -> List[str]:
+        return [detail or name for name, ok, detail in self.checks if not ok]
+
+    @property
+    def ok(self) -> bool:
+        return all(ok for _name, ok, _detail in self.checks)
+
+    @property
+    def exit_code(self) -> int:
+        return 0 if self.ok else 1
+
+    def add(self, name: str, ok: bool, detail: str = "") -> None:
+        self.checks.append((name, ok, detail))
+
+    def warn(self, message: str) -> None:
+        self.warnings.append(message)
+
+    def to_dict(self) -> Dict[str, Any]:
+        return {
+            "ok": self.ok,
+            "checks": [
+                {"name": name, "ok": ok, "detail": detail}
+                for name, ok, detail in self.checks
+            ],
+            "warnings": list(self.warnings),
+        }
+
+
+# ─── Static checks ───────────────────────────────────────────────────────────
+
+
+def _requires_hermes_spec_valid(spec: str) -> bool:
+    """Strictly validate a ``requires_hermes`` spec.
+
+    Unlike :func:`hermes_cli.plugins._version_satisfies` (permissive at load
+    time), validation REJECTS clauses whose version segment doesn't parse —
+    a typo'd spec should fail admission, not silently gate nothing.
+    """
+    from hermes_cli.plugins import _VERSION_COMPARATOR_RE, _version_tuple
+
+    for clause in spec.split(","):
+        clause = clause.strip()
+        if not clause:
+            continue
+        m = _VERSION_COMPARATOR_RE.match(clause)
+        target = m.group(2) if m else clause
+        if _version_tuple(target) is None:
+            return False
+    return True
+
+
+def _check_manifest_fields(report: ValidationReport, manifest: dict) -> None:
+    missing = [
+        f for f in ("name", "version", "description") if not manifest.get(f)
+    ]
+    if missing:
+        report.add(
+            "manifest fields",
+            False,
+            f"plugin.yaml missing required field(s): {', '.join(missing)}",
+        )
+    else:
+        report.add("manifest fields", True, "name, version, description present")
+
+
+def _check_requires_hermes(report: ValidationReport, manifest: dict) -> None:
+    spec = str(manifest.get("requires_hermes") or "").strip()
+    if not spec:
+        report.add("requires_hermes", True, "not declared")
+        return
+    if _requires_hermes_spec_valid(spec):
+        report.add("requires_hermes", True, f"spec {spec!r} parses")
+    else:
+        report.add(
+            "requires_hermes",
+            False,
+            f"requires_hermes spec {spec!r} does not parse "
+            "(expected e.g. \">=0.19\" or \">=0.19, <1.0\")",
+        )
+
+
+def _check_config_spec(report: ValidationReport, manifest: dict) -> None:
+    raw = manifest.get("config")
+    if raw in (None, [], {}):
+        report.add("config spec", True, "not declared")
+        return
+    problems: List[str] = []
+    if not isinstance(raw, list):
+        problems.append("config: must be a list of mappings")
+    else:
+        for i, item in enumerate(raw):
+            if not isinstance(item, dict) or not item.get("key"):
+                problems.append(f"config[{i}]: must be a mapping with a 'key'")
+                continue
+            typ = item.get("type")
+            if typ is not None and str(typ) not in _CONFIG_TYPES:
+                problems.append(
+                    f"config[{i}] ({item['key']}): type must be one of "
+                    f"{'/'.join(sorted(_CONFIG_TYPES))}"
+                )
+            secret = item.get("secret")
+            if secret is not None and not isinstance(secret, bool):
+                problems.append(
+                    f"config[{i}] ({item['key']}): secret must be a boolean"
+                )
+    if problems:
+        report.add("config spec", False, "; ".join(problems))
+    else:
+        report.add("config spec", True, "shape valid")
+
+
+def _check_requires_env(report: ValidationReport, manifest: dict) -> None:
+    raw = manifest.get("requires_env") or []
+    problems: List[str] = []
+    if not isinstance(raw, list):
+        problems.append("requires_env: must be a list")
+        raw = []
+    for i, entry in enumerate(raw):
+        if isinstance(entry, str):
+            name = entry
+        elif isinstance(entry, dict):
+            name = str(entry.get("name") or "")
+        else:
+            problems.append(f"requires_env[{i}]: must be a string or mapping")
+            continue
+        if not _UPPER_SNAKE_RE.match(name):
+            problems.append(
+                f"requires_env[{i}]: {name!r} is not UPPER_SNAKE_CASE"
+            )
+    if problems:
+        report.add("requires_env", False, "; ".join(problems))
+    else:
+        report.add("requires_env", True, "all entries UPPER_SNAKE")
+
+
+# ─── Capability probe (subprocess-isolated) ──────────────────────────────────
+
+# Self-contained harness run in a scratch child process. Imports the plugin
+# module using the same file-location mechanics PluginManager uses, calls
+# register() against a recording stub ctx, and prints a sentinel-prefixed
+# JSON line of what was actually registered. Deliberately imports NOTHING
+# from hermes so a hostile plugin only sees a bare interpreter.
+_PROBE_SCRIPT = r"""
+import importlib.util
+import json
+import sys
+
+plugin_dir = sys.argv[1]
+sentinel = sys.argv[2]
+
+recorded = {"tools": [], "hooks": [], "middleware": [], "commands": []}
+
+
+class RecordingContext:
+    plugin_config = {}
+    profile_name = "default"
+
+    def register_tool(self, name, *args, **kwargs):
+        recorded["tools"].append(str(name))
+
+    def register_hook(self, hook_name, callback):
+        recorded["hooks"].append(str(hook_name))
+
+    def register_middleware(self, kind, callback):
+        recorded["middleware"].append(str(kind))
+
+    def register_command(self, name, *args, **kwargs):
+        recorded["commands"].append(str(name))
+
+    def register_cli_command(self, name, *args, **kwargs):
+        recorded["commands"].append(str(name))
+
+    def __getattr__(self, _name):
+        # Any other registration surface (platforms, providers, skills,
+        # context engines, ...) is accepted as a no-op — the probe only
+        # audits the declared-capability categories.
+        def _noop(*args, **kwargs):
+            return None
+
+        return _noop
+
+
+def emit(payload):
+    print(sentinel + json.dumps(payload))
+
+
+try:
+    spec = importlib.util.spec_from_file_location(
+        "hermes_validate_probe_plugin",
+        plugin_dir + "/__init__.py",
+        submodule_search_locations=[plugin_dir],
+    )
+    module = importlib.util.module_from_spec(spec)
+    module.__path__ = [plugin_dir]
+    sys.modules[spec.name] = module
+    spec.loader.exec_module(module)
+except Exception as exc:
+    emit({"error": "import failed: %s" % exc})
+    sys.exit(0)
+
+register = getattr(module, "register", None)
+if register is None:
+    emit({"error": "no register() function"})
+    sys.exit(0)
+
+try:
+    register(RecordingContext())
+except Exception as exc:
+    emit({"error": "register() raised: %s" % exc})
+    sys.exit(0)
+
+emit(recorded)
+"""
+
+
+def _run_capability_probe(plugin_dir: Path) -> Tuple[Optional[dict], str]:
+    """Run the recording probe in a scratch subprocess.
+
+    Returns ``(recorded, error)`` — exactly one is meaningful: *recorded*
+    is the ``{tools, hooks, middleware, commands}`` dict on success, and
+    *error* is a human-readable failure description otherwise.
+    """
+    with tempfile.TemporaryDirectory(prefix="hermes-validate-") as scratch:
+        env = dict(os.environ)
+        env["HERMES_HOME"] = scratch
+        try:
+            result = subprocess.run(
+                [
+                    sys.executable,
+                    "-c",
+                    _PROBE_SCRIPT,
+                    str(plugin_dir),
+                    _PROBE_SENTINEL,
+                ],
+                capture_output=True,
+                text=True,
+                timeout=_PROBE_TIMEOUT,
+                env=env,
+            )
+        except subprocess.TimeoutExpired:
+            return None, f"capability probe timed out after {_PROBE_TIMEOUT}s"
+
+    payload: Optional[dict] = None
+    for line in (result.stdout or "").splitlines():
+        if line.startswith(_PROBE_SENTINEL):
+            try:
+                payload = json.loads(line[len(_PROBE_SENTINEL):])
+            except json.JSONDecodeError:
+                payload = None
+
+    if payload is None:
+        err = (result.stderr or "").strip()
+        return None, (
+            "capability probe produced no result "
+            f"(exit {result.returncode})" + (f": {err}" if err else "")
+        )
+    if "error" in payload:
+        return None, str(payload["error"])
+    return payload, ""
+
+
+def _declared_list(manifest: dict, key: str) -> List[str]:
+    raw = manifest.get(key) or []
+    if not isinstance(raw, list):
+        return []
+    return [str(item) for item in raw if isinstance(item, str)]
+
+
+def _check_capabilities(
+    report: ValidationReport, manifest: dict, plugin_dir: Path
+) -> Optional[dict]:
+    """Probe actual registrations and diff against declared capabilities.
+
+    Returns the recorded dict (for the built-in collision check) or None
+    when the probe failed / was skipped.
+    """
+    if not (plugin_dir / "__init__.py").is_file():
+        report.warn(
+            "no __init__.py — capability probe skipped (manifest-only plugin)"
+        )
+        report.add("capability probe", True, "skipped (no __init__.py)")
+        return None
+
+    recorded, error = _run_capability_probe(plugin_dir)
+    if recorded is None:
+        report.add("capability probe", False, error)
+        return None
+    report.add("capability probe", True, "register() ran in isolation")
+
+    for kind, manifest_key in (
+        ("tools", "provides_tools"),
+        ("hooks", "provides_hooks"),
+        ("middleware", "provides_middleware"),
+    ):
+        declared = set(_declared_list(manifest, manifest_key))
+        actual = set(recorded.get(kind) or [])
+        undeclared = sorted(actual - declared)
+        unregistered = sorted(declared - actual)
+        if undeclared:
+            report.add(
+                f"declared {kind}",
+                False,
+                f"undeclared {kind} registered (not in {manifest_key}): "
+                f"{', '.join(undeclared)}",
+            )
+        else:
+            report.add(f"declared {kind}", True, "matches registrations")
+        if unregistered:
+            report.warn(
+                f"{manifest_key} declares {', '.join(unregistered)} "
+                f"but register() did not register them"
+            )
+    return recorded
+
+
+def _builtin_tool_names() -> List[str]:
+    """Return the built-in tool registry names (discovery-timing safe).
+
+    ``tools.registry`` starts empty — built-in tool modules self-register on
+    import, so we must run ``discover_builtin_tools()`` first (idempotent;
+    see the AGENTS.md discover_plugins timing pitfall).
+    """
+    try:
+        from tools.registry import discover_builtin_tools, registry
+
+        discover_builtin_tools()
+        return list(registry.get_all_tool_names())
+    except Exception:
+        return []
+
+
+def _check_builtin_collisions(
+    report: ValidationReport, manifest: dict, recorded: Optional[dict]
+) -> None:
+    candidate_tools = set(_declared_list(manifest, "provides_tools"))
+    if recorded:
+        candidate_tools.update(recorded.get("tools") or [])
+    if not candidate_tools:
+        report.add("built-in tool collisions", True, "no tools to check")
+        return
+    builtin = set(_builtin_tool_names())
+    collisions = sorted(candidate_tools & builtin)
+    if collisions:
+        report.add(
+            "built-in tool collisions",
+            False,
+            "tool name(s) collide with built-in tools: "
+            f"{', '.join(collisions)}",
+        )
+    else:
+        report.add("built-in tool collisions", True, "no collisions")
+
+
+# ─── Entry point ─────────────────────────────────────────────────────────────
+
+
+def validate_plugin_dir(plugin_dir: Path) -> ValidationReport:
+    """Run every admission check against *plugin_dir* and return the report."""
+    report = ValidationReport()
+    plugin_dir = Path(plugin_dir)
+
+    if not plugin_dir.is_dir():
+        report.add(
+            "plugin directory", False, f"{plugin_dir} is not a directory"
+        )
+        return report
+
+    manifest_file = plugin_dir / "plugin.yaml"
+    if not manifest_file.is_file():
+        manifest_file = plugin_dir / "plugin.yml"
+    if not manifest_file.is_file():
+        report.add("manifest", False, "no plugin.yaml in the plugin directory")
+        return report
+
+    import yaml
+
+    try:
+        manifest = yaml.safe_load(
+            manifest_file.read_text(encoding="utf-8")
+        )
+    except Exception as exc:
+        report.add("manifest", False, f"plugin.yaml failed to parse: {exc}")
+        return report
+    if not isinstance(manifest, dict):
+        report.add("manifest", False, "plugin.yaml must be a mapping")
+        return report
+    report.add("manifest", True, "plugin.yaml parses")
+
+    _check_manifest_fields(report, manifest)
+    _check_requires_hermes(report, manifest)
+    _check_config_spec(report, manifest)
+    _check_requires_env(report, manifest)
+    recorded = _check_capabilities(report, manifest, plugin_dir)
+    _check_builtin_collisions(report, manifest, recorded)
+    return report